X-ray tubehead



April 1950 J. LEMPERT ET AL 2,504,706

X-RAY TUBEHEAD Filed March 18, 1948 INVENTORS c' reo/vox 19 I ,4 EMPEB? ATTORNEY Patented Apr. 18, 1950 UNITED STATES PATENT OFFICE X-RAY TUBEHEAD Joseph Lempert, West Caldwell, and Robert V.

Gauthier, Glen Ridge, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 18, 1948, Serial No. 15,714

2 Claims.

The present invention relates to X-ray apparatus and more particularly to a ray-proof housing encasing an air cooled X-ray tube.

In such a construction the X-ray tube encased within the housing is supplied with a high voltage which may range from fifty to two hundred kilovolts. The housing is naturally formed of an insulating material normally capable of preventing spark-over between parts of opposite instantaneous polarity and is provided with a window for the egress of X-rays generated by the tube during operation. The main difficulty with apparatus of this type resides in the fact that, while the housing is initially of suiiicient dielectric strength to prevent spark-over, its resistance reduces due to the heat generated by the tube during operation with the result that corona forms across the anode end piece to the copper radiator rod or across the air space between the radiator and the housing. This corona causes a deterioration of the housing which eventually results in its failure as an insulator.

It is accordingly the object of the present invention to provide a ray-proof X-ray housing wherein the high voltage gradient across the anode end piece of the X-ray tube and the air space is reduced so that the formation of corona is eliminated.

Another object of the present invention is the provision of a ray-proof housing for an X-ray tube wherein the voltage gradient between the anode end piece and the air gap between other high voltage carrying parts is reduced by an insulating covering so as to eliminate the formation of corona.

Still further objects of the present invention will become obvious to those skilled in the art by reference to the accompanying drawing wherein:

Figure l is a cross-sectional view of a rayproof housing for an X-ray tube constructed in accordance with the present invention,

Figure 2 is a cross-section View taken on the line II-II of Figure 1 and looking in the direc tion indicated by the arrows,

Figure 3 is a schematic illustration showing one method of applying a semi-conducting coating to the anode end piece to reduce the voltage gradient across the air-space between the anode rod, and the ray-proof housing, and between the housing and cooling radiator.

Referring now to the drawings in detail, there is shown in Figure 1 an X-ray tube 5, supported at its cathode end 6 and anode end 1 within a ray-proof housing 8, provided with a cone-shaped opening 9 and constituting a Window for the egress of the X-ray beam from the tube. As is customary, the housing 8 is formed of an insulating material to prevent spark-over between high potential carrying parts, and is impregnated or lined with a material of high atomic weight, such as lead, to substantially prevent the egress of X-rays through the housing except through the window 9. The X-ray tube 5 is inserted into the housing by sliding the cathode end 6 interiorly thereof until the cathode end enters a loose fitting opening In which thus disposes the base !2 exteriorly of the housing to facilitate fastening of the conductor terminals to the cathode.

The anode end 1 is supported by a split insulating member or anode end piece l3, of higher dielectric strength and higher temperature resistance than the remainder of the housing 8, such as lava, porcelain, or other ceramic material, and constituting an end wall of the housing 8. This anode end wall I3 is provided with an opening M for the copper anode stem l5 and has a plurality of small openings M5 for the air circulation through the housing around the opening it at the cathode end. The anode end wall !3 is held firmly in place by a leaf-spring I1 bearing against the rear surface thereof and carried by the anode stem I5 and by an annular insulating collar l8 threadedly engaging the end of the housing 8. For the purpose of cooling the anode l of the X-ray tube 5 during operation, a cooling radiator 19 telescopically engages the copper anode rod l5. During operation a high potential is impressed across the X-ray tube 5 by having its cathode 6 and anode I connected by cables extending from the base terminal I2 and radiator 59 to the high voltage secondary window of a transformer (not shown).

During normal operation, the housing 8 substantially prevents the egress of X-rays from the X-ray tube 5 except through the window 9, due to the impregnation or lining of high atomic weight material, and the housing has sufiicient dielectric strength to prevent spark-over between high voltage carrying parts. However, when the X-ray tube is operated for an appreciable period of time, either ontinuously or intermittently with short intervals therebetween, a part of the heat generated by the X-ray tube is transmitted by radiation and convention to the housing despite the provision of the cooling radiator l9. Upon a rise in temperature of the housing there follows a decrease in its resistance together with a change in the voltage gradient with the result that, in the absence of provisions to the contrary,

a corona discharge occurs between the copper anode stem l5 and the housing 3 following the surface of the anode end wall it, or across the air-gap between the housing 8 and the cooling radiator l9. Such corona discharge eventually causes deterioration of the housingto such an extent that it finally fails as an insulator for the high voltage impressed across the X-ray tube.

To eliminate the formation of this corona dis-' charge the anode end wall I3 is coated with a semi-conducting material 20 which has a suiiiciently high surface resistance to prevent the housing from being subjected to too .high a voltage yet maintains the voltage gradient across the anode end wall l3 more or less uniform and maintains the voltage below that at which a corona discharge would otherwise occur across the air-gap between the anod end wall it and the housing 8 and radiator l9. This semi-conducting material 20 may be painted or sprayed on to the surface of the anode end wall 13 although we prefer to dip'each half of the latter in a tank such as'shown in Figure 3. Moreover, while several types of semi-conducting material maybe employed, such as a colloidal suspension of graphite in water and sold under the trade name of Aquadag, or glazes containing various metallic oxides to make them semi-conducting, we prefer to use a semi-conducting varnish,

such as that known under the trade name of Coronox. Such varnish is diluted with a solvent until its viscosity is sufliciently low that when contained in a tank, as shown in Figure 3, the halves of the anode end wall I3 can be dipped therein by supporting 'thepiece 0n ahook or the like '22 and thereafter dried 'prior to use by baking for approximately one half an hour at about 100 C.

It should thus .become obvious to those skilled in the art that a ray-proof .X-ray tube housing h is provided by the present invention which prevents the emission of X-rays except through a window provided in the housing. Moreover, the

housing is provided'witha semi-conducting coating at the points where a corona discharge would otherwise occur which completely eliminates the formation of such a corona discharge and prevent deterioration of the dielectric constantof the'ray-pro of housing.

Although a specific embodiment of the present invention :has been'shown and described, it will be understood that still further modifications thereof may be made without departing from the spirit and scope of the appended claims.

We claim:

1. A ray-proof X-ray tube unit'comprising an between high voltage carryingparts of said unit,

an insulating end wall for saidhousing having a higher dielectric strength and temperature resistance than the remainder of said housing, an annular collar of insulating material engageable with said housing for retaining said insulating end wall in position within said housing, an anode cooling rod for said X-ray tube projecting 'exteriorly of said housing through the end wall thcrern for supporting a cooling radiator, a spring disposed between said anode cooling rod and said insulating end wall and cooperating with said annular collar in retaining said insulating end wall within said housing, and a semi-conducting coating on the surface of said insulating end wall between said anode cooling rod and the remaining surface of said housing to distribute the potential gradient substantially uniformly across said insulating end wall and to maintain the voltbelow that at which a corona discharge would otherwise occur, thus preventing the formation of such discharge between said radiator or said anode cooling rod, and the remainder of said housing.

'2. A'ray-proof X-ray tube unit comprising an X-ray tube, a housing surrounding said X-ray tube provided with a window therein for pre- .".venting the emission of X-rays from said tube except through said window and to prevent spark-over between high voltage carrying parts of said unit, an insulatingend wall for said housing having a higher dielectric strength and temperature resistance than the remainder of said housing, an anode cooling rodfor said X-ray tube projecting exteriorly of said housing through the end wall thereof for supporting a cooling radiator, a spring disposed between the anode cooling rod and said insulating end wall for retain- :ing the latter in place within said housing, and

a thin coating of a semi-conducting material on the surface of said insulating end wall between said anode cooling rod and the remaining surface of said housing to distribute the potential gradient substantially uniformly across said in sulating end wall and to maintain the voltage below that at which a corona discharge would otherwise occur, thus preventing the formation of such discharg between saidradiator or said anode cooling rod, and the remainder of said housing.

JOSEPH LEMPERT. ROBERT V. .GAUTHIER.

REFERENCES CITED "he io'ilowing references are of record in file of this patent:

UNITED STATES PATENTS Number Name 1,693,603 Holden Oct. 19, 1926 1,995,150 Kaplan Man 19, 1935 2,132,174 Machlett Oct. 4, 1938 2,256,229 Atlee ct al. Sept. 16, 1941 2,318,074 Hill et al May 4, 1943 

